Introduced as part of the BTC's pilot scheme
locomotives, an initial batch of 4 machines was rapidly increased to an
order of 52 for use on passenger and freight work in the north London
suberbs. Early engine problems soon led to the whole class being
transferred to Scotland for more suitable work, and moreover, closer to
their North British Locomotives builders workshops where rectification
work could be carried out locally. The engine problems persisted however,
which finally resulted in twenty of the class re-engined with a Paxman
Ventura power unit. These modified Class 29 machines where recognisable by
the replacement of the disc headcodes with standard four-character route
indicator blinds. Withdrawals commenced in December 1967 with the final
locomotives, 6112/16/19/33, finally removed from the BR network in
Engine: Class 21: NBL/MAN 12-cyl L12V18/21S, pressure-charged of 1000 or 1100bhp (several machines de-rated). Class 29: Paxman 12YJXL of 1350bhp.
Numbers: (D)6100 - (D)6157.